Methods and Devices for Uplink Diversity Transmission

ABSTRACT

A user equipment transmits using at least two uplink transmit antennas and receives a set of control signals in the downlink direction from a cellular network. The user equipment estimates a received signal quality for each control signal in said set of control signals and determines, based on said received signal quality, which control signals have been reliably received. The user equipment derives one or more parameters related to the uplink transmit diversity operation using a subset of control signals from the set of control signals, said subset only including control signals determined as reliably received; and transmits in the uplink direction applying the derived one or more parameters to control the uplink transmit diversity operation. The accuracy of the transmit diversity parameter values derived/set by the UE can be improved. This will enhance the performance of the uplink transmit diversity and will also reduce interference to the neighbor cells.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/499,957, filed 3 Apr. 2014, which was a national stage entryunder 35 U.S.C. §371 of international patent application Serial No.PCT/SE2010/051067, filed 4 Oct. 2010, which claimed the benefit of U.S.provisional application Ser. No. 61/250,063, filed 9 Oct. 2009. Theentire contents of each of these applications are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to methods and devices for controllinguplink diversity transmission in a radio system.

BACKGROUND

A study item called ‘Uplink transmit diversity for HSPA’, see RP-090987,has recently been completed in 3GPP. The objective of the study item wasto perform the feasibility study to evaluate the system gain of uplinktransmit diversity schemes for High Speed Packet Access (HSPA) as wellas analyze the impact on the User equipment (UE) and base stationimplementation and complexity in a cellular radio system.

The baseline and typical UE implementation comprises a single uplinktransmit antenna used for all types of uplink transmission. However,high end UEs may have and use multiple uplink transmit antennas foruplink transmission. This is commonly referred to as uplink transmitdiversity. The objective of the transmit diversity transmission is toachieve higher uplink data rate and/or lower UE transmission power bythe virtue of spatial, angular and/or temporal diversities.

A commonly used uplink transmit diversity consists of two uplinktransmit antennas. The signals from two or more uplink transmitdiversity antennas may be transmitted in different manner in terms oftheir phases, amplitude, power etc. This gives rise to different uplinktransmit diversity schemes. Some well known schemes are:

-   -   Transmit beamforming open loop    -   Transmit beamforming closed loop    -   Switched antenna uplink transmit diversity open loop    -   Switched antenna uplink transmit diversity closed loop    -   Space time transmit diversity

Typically, in any transmit diversity scheme, a set of parameters relatedto uplink transmit diversity are regularly adjusted by the UE. Theobjective is to ensure that the uplink transmission incorporates thedesired spatial, temporal or angular diversities. This in turns improvesuplink coverage, reduces interference, increases uplink bit rate andenables UE to lower its transmitted power.

The transmit diversity parameters may comprise of one or many of:relative phase, relative amplitude, relative power, relative frequency,timing, absolute or total power of signals transmitted on transmitdiversity branches, etc.

The adjustment of all or a sub-set of these parameters is fundamental totransmit beamforming scheme. The objective of beamforming is to directthe uplink transmission or beam towards the desired base station, whichis generally the serving base station. This allows the serving basestation to decode the received signal more easily. Furthermore, highdirectivity of the beam towards the desired base station reduces theinterference towards the neighboring base stations.

Similarly in case of switched antenna transmit diversity, transmitdiversity parameter implies the selection of the most appropriatetransmit antenna (e.g., in terms of radio condition) out of theavailable transmit diversity branches. By the virtue of using the mostappropriate antenna for the uplink transmission, the UE can eitherreduce its power while retaining a given uplink information rate, orincrease the information rate while retaining a given output power.

In open loop transmit diversity schemes, the UE autonomously adjusts theuplink transmit diversity parameters without the use of any networktransmitted control signaling or commands. These schemes are simpler,although they may not show substantial gain in all scenarios.

On the other hand in closed loop transmit diversity schemes, the UEadjusts the uplink transmit diversity parameters by making use of asuitable network transmitted control signaling or commands. Thesecommands or control signals reflect the uplink quality e.g., the qualitymeasured at the base station. These commands (control signals) aresignaled to the UE over the downlink. Furthermore, the commands can besent exclusively to the UE to enable it to adjust the uplink transmitdiversity parameters. Alternatively the UE can utilize any existingcommands or signaling, which are originally intended for other purposes,to derive the uplink transmit diversity parameters. Examples of suchimplicit signaling or commands are transmit power control (TPC) commandsand Hybrid Automatic Repeat Request (HARQ) Acknowledgement/NegativeAcknowledgement (ACK/NACK) etc which are sent to the UE by the basestation for uplink power control and uplink HARQ retransmission schemerespectively. The closed loop schemes have a potential of leading to abetter performance gain due to the use of network controlled signalingfor adjusting the uplink transmit diversity parameters.

Furthermore transmit diversity schemes can be used in any cellular radiosystem technology including Long Term Evolution (LTE), Wideband CodeDivision Multiple Access (WCDMA) or Global System for MobileCommunication (GSM). For instance in LTE, the switched antenna uplinktransmit diversity is standardized in LTE release 8.

Reliability is particularly important in transmissions where an entiredecision relies upon one or more simple commands such as on/offsignaling, ACK and NACK responses, up and down power control commands,etc. Unreliable commands may cause actions to occur in reverse directionand in some cases may lead to unstable behavior.

The unreliability generally occurs due to bad radio conditions, lowtransmitted power level, poor coverage, high system load etc whereby thereceiver cannot properly interpret the correct meaning of the command.

To ensure reliable operation, four sets of functionalities can bespecified:

-   -   Reliability check    -   Behavior or action of UE and/or base station    -   Indication or reporting of unreliability events to network    -   Prevention of unreliability

The reliability check can be based either on some signal strength orquality such as signal to interference ratio (SIR). It can also be basedon some bit error rate (BER) target value. This means a received commandis regarded as unreliable in case the received signal quality orstrength falls between the thresholds or if the BER is higher than thetarget.

The behavior of UE or base station in response to unreliable commanddetection depends upon a particular functionality governed by the on/offcommand.

A number of uplink transmit diversity schemes exists which use networkcontrol signaling or commands to derive the uplink transmit diversityparameters. For instance certain schemes such as uplink transmitbeamforming could make use of TPC commands or HARQ ACK/NACK sent by thebase station to derive the uplink transmit diversity parameters.Similarly the switched antenna uplink transmit diversity scheme couldmake use of TPC commands or HARQ ACK/NACK for the antenna selection.Both these schemes have been studied within the scope of the recentlycompleted study item in 3GPP, see RP-090987.

The above signaling (TPC or HARQ ACK/NACK or the like), which aretransmitted on the downlink, are used to characterize the uplinkquality. However due to poor downlink quality the commands or networksignaling such as TPC can be erroneously received by the UE. Forinstance due to error the TPC, which is either 0 (e.g., DOWN) or 1(e.g., UP), if transmitted as 1 can be interpreted as 0 or vice versa.This will lead to erroneous adjustment of transmit diversity parametersin case TPC (or similar commands or signaling) is used for this purpose.The erroneous adjustment of parameters may lead to incorrect beamformingor antenna switching. For instance the beam may be directed to theneighbor base station instead of the serving one. The uplink performance(i.e., UE with erroneous parameter setting due to incorrect receivedcommand) will deteriorate and the neighboring cells will receiveexcessive and unnecessary interference.

Further, US patent application No. 2008/123768 describes a system whereTPC and HARQ indicator (ACK/NACK) for setting weighting of signalsbetween the 2 transmit antennas. The HARQ quality indicator is not usedwhen NACK is received.

The downlink quality varies with the radio conditions and particularlydeteriorates for UEs which are close to the cell border or when thedownlink load is high. Especially when being close to the cell borderthe UE is generally closer to the neighboring base stations and far fromthe serving one. Hence, the erroneously received commands (controlsignals), which cause incorrect setting of the uplink diversityparameters for the uplink transmit diversity transmission, will lead toeven more substantial interference and lower performance.

It is therefore important to develop methods and arrangements, which canensure that the values of the uplink transmit diversity parameters arecorrectly derived.

SUMMARY

It is an object of the present invention to provide an improved methodand device to address the problems as outlined above. This object andothers are obtained by the methods and devices as set out in theappended claims.

In accordance with one embodiment a method in a user equipment forcontrolling uplink transmit diversity operation is provided. The userequipment transmits using at least two uplink transmit antennas andreceives a set of control signals in the downlink direction from acellular network. The user equipment estimates a received signal qualityfor each control signal in said set of control signals and determines,based on said received signal quality, which control signals that havebeen reliably received. The user equipment derives one or moreparameters related to the uplink transmit diversity operation using asubset of control signals from the set of control signals, said subsetonly including control signals determined as reliably received; andtransmits in the uplink direction applying the derived one or moreparameters to control the uplink transmit diversity operation. Theaccuracy of the transmit diversity parameter values derived/set by theUE can be improved. This will enhance the performance of the uplinktransmit diversity and will also reduce interference to the neighborcells.

In accordance with one embodiment the control signals includes one ormore types of the following control signals: transmit power controlcommands sent to the user equipment for controlling uplink transmitpower of the user equipment; Hybrid Automatic Repeat Request, HARQ,Acknowledgement/Negative Acknowledgement, ACK/NACK, signals sent to theuser equipment for controlling HARQ retransmissions by the userequipment; and control signals for explicit control of the userequipment transmit diversity parameters.

In accordance with one embodiment, the received signal quality isestimated based on one or more out of: Signal to interference ratio,SIR; Signal to interference plus noise ratio SINR; bit error rate; blockerror rate; service data unit (SDU) error rate; frame error rate; andsymbol error rate.

In accordance with one embodiment the set of control signals are allreceived on a single radio link. In accordance with one embodiment theset of control signals are received on multiple radio links.

In accordance with one embodiment the one or more parameters compriseone or more of the following, for signals transmitted on transmitdiversity branches: relative phase; relative power; relative frequency;timing; relative amplitude; and absolute power.

In accordance with one embodiment the uplink transmit diversityoperation is transmit diversity beamforming and the one or moreparameters comprise relative phase and amplitude of signals transmittedon transmit diversity branches. In accordance with one embodiment theuplink transmit diversity operation is switched antenna uplink transmitdiversity and the one or more parameters comprise amplitude or power ofsignals transmitted on transmit diversity branches.

In accordance with one embodiment if no control signal in said set ofcontrol signals is determined as reliably received, the user equipmentperforms uplink transmissions using less than all transmit antennas insaid set of at least two uplink transmit antennas.

In accordance with one embodiment if no control signal in said set ofcontrol signals is determined as reliably received, the user equipmentswitches to a single transmit antenna transmission mode in which uplinktransmissions are performed using a single antenna in said set of atleast two uplink transmit antennas.

In accordance with one embodiment if the set of control signals consistsof a number of N control signals out of which less than a number of Mcontrol signals are determined as reliably received, M<N, the userequipment performs uplink transmissions using less than all transmitantennas in said set of at least two uplink antennas.

In accordance with one embodiment if the set of control signals consistsof a number of N control signals out of which less than a number of Mcontrol signals are determined as reliably received, M<N, the userequipment switches to a single transmit antenna transmission mode inwhich uplink transmissions are performed using a single antenna in saidset of at least two uplink transmit antennas.

According to one embodiment, a method in a user equipment (UE) isprovided for controlling uplink transmit diversity operation accordingto which the UE transmits using at least two uplink transmit antennas. Aset of control signals is received in the downlink direction by the UEfrom a cellular network. The UE estimates received signal quality foreach control signal in said set of control signals and determines, basedon said received signal quality, which control signals have beenreliably received. The UE derives one or more parameters related to theuplink transmit diversity operation using a subset of control signalsfrom said set of control signals, said subset only including controlsignals determined as reliably received. The UE transmits in the uplinkdirection while applying the derived one or more parameters to controlthe uplink transmit diversity operation.

According to one embodiment a method in a cellular network is providedfor interacting with a UE performing uplink transmission using an uplinktransmit diversity operation scheme. The cellular network transmits acommand signal to the UE, said command signal ordering the UE to reportto the network when the UE for a defined period of time has failed toappropriately derive one or more parameters related to the uplinktransmit diversity operation from control signals received by the UE inthe downlink direction from the network. The failure to appropriatelyderive said parameters being due to low signal quality of said controlsignals as received by the UE. The cellular network further receives areport from the UE that it has failed to appropriately derive one ormore parameters related to the uplink transmit diversity operation forthe defined period of time.

According to one embodiment, a method in a cellular network is providedfor interacting with a UE performing uplink transmission using an uplinktransmit diversity operation scheme. The cellular network (and inparticular one or more base stations) monitors uplink transmissions fromthe UE for detecting events indicating that the UE potentially havefailed to appropriately derive one or more parameters related to theuplink transmit diversity operation from control signals received by theUE in the downlink direction from the network.

Embodiments can include different methods in UE and network as well asrespective apparatuses (UE and network nodes such as radio basestations) configured to, i.e., comprising means adapted to, implementthese different methods.

The invention also extends to User Equipments arranged to control uplinktransmission in accordance with the above methods. To enable the UserEquipment to perform uplink transmission in accordance with the abovemethods, the user equipment can be provided with a controller (orseveral controllers) having circuitry arranged to perform the aboveprocesses. The controller(s) can be implemented using suitable hardwareand or software. The hardware can comprise one or many processors thatcan be arranged to execute software stored in a readable storage media.The processor(s) can be implemented by a single dedicated processor, bya single shared processor, or by a plurality of individual processors,some of which may be shared or distributed. Moreover, a processor or mayinclude, without limitation, digital signal processor (DSP) hardware,ASIC hardware, read only memory (ROM), random access memory (RAM),and/or other storage media.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way ofnon-limiting examples and with reference to the accompanying drawing, inwhich:

FIG. 1 is a general view of a cellular radio system,

FIG. 2 is a view of a user equipment,

FIG. 3 illustrates the relation between the TPC SIR and TPC symbol errorrate,

FIG. 4 is a flowchart illustrating some procedural steps performed whencontrolling uplink transmission in a user equipment,

DETAILED DESCRIPTION

The invention is applicable to a UE, alternatively referred to as amobile station, which is capable of transmitting any type of signal suchas data, control information, higher or lower layer signaling etc, usingmore than one transmit antenna. This capability is generally called‘uplink transmit diversity’. A typical uplink transmit diversityimplementation may comprise two transmit antennas. However, theimplementation is not limited to two antennas; it may comprise more thantwo antennas. One of the antennas may be regarded as the primaryantenna, which corresponds to the baseline configuration, whichtypically comprises a single transmit antenna. The remaining transmitantennas can be regarded as the secondary antennas corresponding to moreadvanced antenna configurations.

In FIG. 1 a general view of a cellular radio system 100 is depicted. Thesystem 100 depicted in FIG. 1 is a WCDMA system and uses terminologyassociated with WCDMA system. It is however understood that theinvention is applicable to any type of cellular radio system such asGSM, LTE and other similar systems. The system 100 comprises a number ofbase stations 101, whereof only one is shown for reasons of simplicity.The base station 101 can connect to user equipments in the figurerepresented by the UE 103 located in the area served by the base station101. The system 100 is arranged to allow the use of uplink transmitdiversity schemes. The base station and the user equipment furthercomprise controller(s) or controller circuitry 105 and 107 for providingfunctionality associated with the respective entities. The controllers105 and 107 can for example comprise suitable hardware and or software.The hardware can comprise one or many processors that can be arranged toexecute software stored in a readable storage media. The processor(s)can be implemented by a single dedicated processor, by a single sharedprocessor, or by a plurality of individual processors, some of which maybe shared or distributed. Moreover, a processor may include, withoutlimitation, digital signal processor (DSP) hardware, ASIC hardware, readonly memory (ROM), random access memory (RAM), and/or other storagemedia.

In FIG. 2 a user equipment 103 adapted to transmit in the uplink using adiversity transmission scheme. The user equipment 103 comprises a numberof transmit antennas 111 whereof two are shown for reasons ofsimplicity. It is understood that the user equipment can comprise anynumber of transmit antennas. The antennas are shown to illustrate thatthe user equipment is capable of uplink transmit diversity. As describedabove the user equipment 103 further comprises one or many controllersor controller circuitry 107 to enable the user equipment to implementthe uplink diversity transmit methods as described herein.

In accordance with one embodiment the UE adjusts one or more parametersrelated to the uplink transmit diversity in order for it to adequatelytransmit on the uplink using transmit diversity. The transmit diversityparameter may comprise one or more of the following: relative phase,relative amplitude, relative power, relative frequency, timing, absoluteor total power of signals transmitted on transmit diversity branches,etc. Relative phase may comprise of the phase difference between thephase of the signal transmitted from the first transmit antenna and thephase of the signal transmitted from the second antenna or vice versa.Relative power in dB scale may comprise of the power difference betweenthe power of the signal transmitted from the first transmit antenna andthe power of the signal transmitted from the second antenna or viceversa. Relative amplitude in dB may comprise of the amplitude differencebetween the amplitude of the signal transmitted from the first transmitantenna and the amplitude of the signal transmitted from the secondantenna or vice versa. Relative frequency may comprise of the frequencydifference between the frequency of the signal transmitted from thefirst transmit antenna and the frequency of the signal transmitted fromthe second antenna or vice versa. Timing means setting appropriatetransmit timing of signals transmitted on the diversity branches.Ideally the transmit timing of signals on all antennas should be thesame. Hence the relative time should be as small as possible. Relativetiming may comprise of the time difference between the transmittedtiming of the signal transmitted from the first transmit antenna and thetransmitted timing of the signal transmitted from the second antenna orvice versa. Therefore similar absolute timings are to be used on alltransmit diversity branches. In that Absolute or total power is thetotal power transmitted from all transmit antennas.

Relative power and relative amplitude may also be expressed in linearscale, in which case they will be ratios of powers or signals from thetransmit antennas.

Different types of uplink transmit diversity schemes or variants can beemployed by using an appropriate algorithm for the adjustment oftransmit diversity parameters. For instance transmit diversityparameters such as relative phase and amplitude can be adjusted tocreate transmit diversity beamforming. Similarly in case of ‘switchedantenna uplink transmit diversity’ the output of the modulator isswitched between the two transmit antennas by the UE such that the UEtransmits using one antenna at a time. This corresponds to the fact thatthe diversity parameter such as amplitude or power from the otherantenna(s) is zero. The invention is therefore applicable to any uplinktransmit diversity scheme, in particular those utilizing some sort ofcontrol signals transmitted by the network in the downlink direction,which can be used for adjusting transmit diversity parameters.

Thus, an adjustment of the transmit diversity parameters can be based onone or more types of signals received by the UE from radio networknode(s). Examples of signals are the: transmit power control (TPC)commands sent by the base station to the UE for controlling the uplinktransmitted power of the UE, HARQ ACK/NACK sent by the base station tothe UE for controlling HARQ retransmissions by the UE as well as anyother explicit signaling for controlling or adjusting the UE transmitdiversity parameters etc.

The following functionalities are elaborated down below:

-   -   Reliability Detection    -   UE Behavior for Reliability Handling

Yet another functionality that can be used is the UE indicating to thenetwork that repeatedly received unreliable commands results or mayresult in an inappropriate derivation of uplink transmit diversityparameter:

-   -   Event reporting and network action

It should be noted that in the subsequent sections, the embodiments aredescribed for the uplink transmit diversity and in particular for thetransmit beamforming and the switched antenna uplink transmit diversityschemes. However, the embodiments are applicable to any type of uplinktransmit diversity scheme, which use some sort of downlink networkcontrol signaling for deriving the values of the uplink transmitdiversity related parameters. Similarly the different methods that aredescribed can be applied to any access technology notably GSM, WCDMA,cdma2000, LTE Frequency Division Duplex (FDD), LTE Time Division Duplex(TDD) or Universal Mobile Telecommunications System Terrestrial RadioAccess Network (UTRAN) TDD etc.

As regards reliability detection, downlink signals such as Transmitpower control (TPC) commands, HARQ ACK/NACK etc received by the UE areused by the UE to derive the values of the uplink transmit diversityparameters. Therefore these downlink control signals (or commands),which are sent by the network, reflect the uplink signal quality.However, these signals as received by the UE in the downlink may beerroneous due to poor downlink quality. For instance due to poordownlink quality, the UP TPC command (e.g., 1) sent by the network maybe interpreted as DOWN (e.g., 0). Hence, on/off type signals withoutstrong protection e.g., in the form of channel coding, which may beeither ‘0’ or ‘1’ such as TPC, HARQ ACK/NACK etc are highly prone toreception errors.

A UE determines the reliability of the received signals before usingthem for deriving or adjusting the transmit diversity parameters orusing them for any transmit diversity related action or decision.

In order to determine the reliability of a set of received signals, theUE may perform the steps of:

-   -   Receiving a set of downlink signals, which can be e.g., TPC        commands or HARQ ACK/NACKs or similar signals transmitted by        network node(s)    -   Measuring, estimating or deriving the downlink quality of the        received signals, wherein the said downlink quality of the        received signals or commands is based on any of the following:        signal strength, SIR, SINR, bit error rate (BER) or symbol error        rate of the said downlink received signal or of any suitable        common or dedicated pilot signal, which can be mapped to the        downlink quality. Examples of common pilot signals are common        pilot channel (CPICH) and synchronization channel (SCH) in UTRAN        or reference signal and SCH in LTE. Examples of dedicated pilots        are the pilot symbols sent on dedicated physical control channel        (DPCCH) in WCDMA or dedicated reference signal (DRS) in LTE.    -   Comparing the received signal quality of the signals or commands        with a threshold to determine whether the downlink received        signals are reliable or not. A received signal is reliable if        its quality is above a threshold, otherwise it is unreliable.        The threshold can be determined by the UE, pre-defined in the        standard or signaled by the network to the UE.

As an example, FIG. 3 illustrates the relation between the TPC SIR andTPC symbol error rate. The UE measures the SIR of the received TPC bitsand compares this with the threshold (γ) to check whether the receivedTPC bits are unreliable or not. For instance in WCDMA, the UE can firstmeasure SIR on the TPC commands sent over Fractional Dedicated PhysicalChannel (F-DPCH) and use a mapping function to derive the TPC commanderror rate. The target TPC error rate can be set to a suitable thresholdlevel for example 30%, beyond which the command can be regarded asunreliable.

Regarding UE behavior for reliability handling, the UE can be adapted touse only the reliably received downlink signals for deriving the uplinktransmit diversity parameter for any uplink transmit diversity decisionsuch as antenna selection for uplink transmission.

In FIG. 4 a flowchart illustrating some steps performed when controllinguplink transmit diversity operation in a UE is depicted. The UE can beusing at least two uplink transmit antennas. First, in a step 401, theUE receives a set of control signals in the downlink direction from acellular network. The UE estimates a received signal quality for eachcontrol signal in said set of control signals in a step 403. Next, in astep 405 then determines, which control signals that have been reliablyreceived based on the received signal quality. Thereupon, the UE derivesone or more parameters related to the uplink transmit diversityoperation using a subset of control signals from said set of controlsignals, said subset only including control signals determined asreliably received in a step 407. The UE then in a step 409 transmits inthe uplink direction while applying the derived one or more parametersto control the uplink transmit diversity operation.

The UE behavior of handling the unreliable commands, most notably theTPC commands, for deriving the uplink transmit diversity parameters orfor antenna selection can be specified in the applicable standard i.e.,pre-determined rule(s) governing the UE behavior. Different criteria forthe UE behavior of handling unreliable commands or control signals canbe specified i.e., pre-determined rule(s).

For example, for any type of uplink transmit diversity, the criteria canbe specified as: The UE shall use only reliable received commands (e.g.,TPC or any explicit signaling for UL transmit diversity) for deriving orsetting the uplink transmit diversity parameters used for uplinktransmit diversity.

In case of transmit beamforming the criteria can be specified as: The UEshall use only reliable received commands (e.g., TPC or any explicitsignaling) for deriving or setting the uplink transmit diversityparameters used for uplink transmit beamforming.

In case of switched antenna transmit diversity the criteria can bespecified as: The UE shall use only reliable received commands (e.g.,TPC or any explicit signaling) for selecting uplink antenna for uplinktransmission in case of switched antenna transmit diversity.

Yet another criterion can be specified as: The UE shall use only asingle transmit antenna or shall not use uplink transmit diversity incase all received commands are unreliably received over a certainperiod.

The UE behavior depends on whether it receives signals or commands froma single radio link or multiple radio links. A radio link is a wirelesslink between the UE and a radio base station over which controlinformation and/or data are exchanged between the UE and the radio basestation. The radio base station can for example be a Node B in UTRAN, aneNode B in E-UTRAN, an access point or the like. Within the same site orlocation there may be more than one radio base station. Typicallytransmission and reception of data/control takes place using a singleradio link. However in some cases like in soft handover multiple radiolinks are used for reception and transmission of data and controlinformation.

In case of a single radio link, when the UE receives downlink signals orcommands from only one radio link. The UE can perform a step of: Usingonly the reliable downlink signals/commands such as TPC or HARQ ACK/NACK(i.e., whose quality is above threshold) for deriving or adjusting oneor more uplink transmit diversity parameters such as relative phase,relative amplitude, relative power, relative frequency, timing, absoluteor total power of signals transmitted on transmit diversity branches orfor the selection of uplink transmit antenna.

According to one embodiment when the UE uses a window (W) comprising ofa number N last received commands to derive the said uplink transmitdiversity parameters, the UE can perform a further step of: Using all M,M being a number equal or less than the number N (M≦N) reliably receivedsignals/commands out of the last N total received signals/commands forderiving uplink transmit diversity parameters or for the selection ofuplink transmit antenna.

In case all N last received commands are unreliable, then the UE canperform a further step of using only a single transmit antenna foruplink transmission until: the expiry of time period (T0), which caneither be determined by the UE or be a pre-defined value or be signaledby the network; or at least one received command out of the last Nreceived commands becomes reliable.

According to another embodiment, if all N last received commands areunreliable then the UE can perform the steps of:

-   -   Extending the size of the window (W) of last received commands        to a number L, where L is larger than the number N (L>N) for        deriving the uplink transmit diversity parameters.    -   Reducing the size of the said window to the pre-defined or        default value when at least a number K commands out of L        commands are reliably received.

In case of multiple radio links, the UE receives downlink signals orcommands from more than one radio link. Each radio link connects the UEto a different cell, which may or may not belong to the same basestation site. This is the case of soft handover in CDMA systems e.g.,WCDMA or cdma2000. Another example is that of coordinated multipointtransmission and reception (CoMP), which allows the UE to receive andtransmit data over multiple radio links. The CoMP can be employed invarious technologies including WCDMA and LTE.

In such a case with multiple radio links, the UE can perform a step of:Using only the reliable downlink commands such as TPC or ACK/NACK (i.e.,whose quality is above threshold) from each radio link (or in otherwords using only the reliable radio link(s)) for deriving or adjustingone or more uplink transmit diversity parameters such as relative phase,relative amplitude, relative power, relative frequency, timing, absoluteor total power of signals transmitted on transmit diversity branches orfor the selection of uplink transmit antenna.

According to one embodiment, when the UE uses a window (W) comprising ofN last received commands from each radio link to derive the said uplinktransmit diversity parameters, the UE can perform a step of: Using all M(MN) reliably received commands from each radio link out of the last Ntotal received commands from each radio link for deriving uplinktransmit diversity parameters or for the selection of uplink transmitantenna.

In case all N last received commands from all the radio links aredetermined to be unreliable, then the UE can perform a step of usingonly a single transmit antenna for uplink transmission until: the expiryof time period (T0), which can either be determined by the UE or be apre-defined value or be signaled by the network; or at least onereceived command out of the last N received commands from at least oneradio link becomes reliable.

According to one embodiment if all N last received commands from all theradio links are determined to be unreliable then the UE can perform thefurther steps of:

-   -   Extending the size of the window (W) of last received commands        to L (L>N) of all the radio links for deriving the uplink        transmit diversity parameters    -   Reducing the size of the said window to the pre-defined or        default value when at least K commands out of L commands for at        least one radio link are reliably received.

The UE can be adapted to ignore unreliable commands to prevent errors inadjusting the transmit diversity parameters. If the received commandsare frequently unreliable, then this may lower the performance of uplinktransmit diversity. The network should preferably be informed (orotherwise detect) when this happens and take appropriate action toremedy the situation.

Hence, according to one embodiment, the UE can be adapted to explicitlyreport to the network when over a period of time the UE is unable toappropriately derive the transmit diversity parameters due to unreliabledownlink, which causes unreliably received commands. This errorcondition can be reported by the UE to the network in the form ofnetwork configured events. The event parameters such as duration (T1)over which certain transmit diversity parameters (P) cannot be properlyadjusted can be configured by the network or can also be pre-defined.

In response to event reporting the network can take appropriate action.Such actions include increasing the transmit power level, congestioncontrol to reduce interference or in the worst case cell change etc.This information can also be used for operational and maintenancepurposes, i.e., for long term network planning to set correcttransmission power levels, cell size dimensioning etc.

Furthermore, the network can be adapted to also use the UE reportedevent (or any relevant error condition regarding the setting ofparameters due to poor downlink quality) to turn off the transmitdiversity altogether by requesting the UE to go to fallback mode i.e.,single transmit antenna transmission mode.

Event reporting as described above explicitly notifies the network aboutthe status of inappropriate setting of parameters for the uplinktransmit diversity due to unreliable received commands. But thisinvolves some signaling overhead due to the event reporting in theuplink. As another alternative, the network can itself infer that theuplink transmit diversity parameters are not correctly set without anyexplicit UE reports or any event reporting.

As an example in case of beamforming, if the beam direction of thereceived beam at the base station deviates by a certain threshold, thenthe network can increase the transmit power of the downlink transmittedsignals or commands. Another possibility is that the network turns ofthe transmit diversity. Yet another possibility is that the networkperforms handover. The handover to another cell may ensure betterdownlink quality, which in turn will improve the reliability of thedownlink received command at the UE.

Similarly due to improper beam directivity the neighboring base stationmay receive higher interference. Based on the uplink cell load (e.g.,served users) and network planning knowledge (e.g., statistics ofinterference), the base station can identify whether the interference isnormal or due to specific cause such as inappropriate beamforming. Sucha base station can also inform neighboring base stations about theexpected interference from their served users.

In accordance with one embodiment a method in a cellular network forinteracting with a UE capable of performing uplink transmission using anuplink transmit diversity operation scheme is provided. In accordancewith the method a command signal is transmitted to the UE, said commandsignal ordering the UE to report to the network when the UE for adefined period of time has failed to appropriately derive one or moreparameters related to the uplink transmit diversity operation fromcontrol signals received by the UE in the downlink direction from thenetwork, the failure to appropriately derive said parameters due to lowreceived signal quality of said control signals as received by the UE.The network receives a report from the UE that it has failed toappropriately derive one or more parameters related to the uplinktransmit diversity operation for the defined period of time.

In accordance with one embodiment the receipt of the report from the UEtriggers the network to perform at least one out of:

-   -   increasing transmit power in the downlink;    -   adapting congestion control to reduce downlink interference;    -   ordering the UE to change cell e.g., by performing handover;    -   ordering the UE to use single antenna uplink transmission.

In accordance with one embodiment the command signal from the network tothe UE and the report from the UE to the network are transmitted using aradio resource control protocol.

Further, in accordance with another embodiment a method in a cellularnetwork for interacting with a UE performing uplink transmission usingan uplink transmit diversity operation scheme is provided. In accordancewith the method, the network monitors uplink transmissions from the UEfor detecting events indicative of the UE potentially having failed toappropriately derive one or more parameters related to the uplinktransmit diversity operation from control signals received by the UE inthe downlink direction from the network.

In accordance with one embodiment the uplink transmit diversityoperation scheme is beamforming and wherein the events include at leastone of:

-   -   received beam direction deviating from an expected direction by        more than a certain threshold;    -   higher than expected received interference at a neighboring base        station.

The methods and user equipments as described herein will improve theaccuracy of the transmit diversity parameter values derived/set by theUE. This will enhance the performance of the uplink transmit diversityand will also reduce interference to the neighbor cells. Also, the UEevent reporting or independent detection at the network of theunreliable reception of the downlink signals at the UE, will enable thenetwork to take appropriate action (e.g., adjusts transmit power etc).As a consequence of the network taking actions to improve the downlink,the accuracy of the transmit diversity parameter values set by the UEcan be improved. This will enhance the performance of the uplinktransmit diversity and will also reduce interference to the neighborcells.

1. A user equipment, comprising: at least two uplink transmit antennas,and controller circuitry, wherein the controller circuitry is configuredto: receive a set of control signals in the downlink direction from acellular network; estimate a received signal quality for each controlsignal in said set of control signals; determine, based on said receivedsignal quality, which control signals have been reliably received;derive one or more parameters related to the uplink transmit diversityoperation using a subset of control signals from said set of controlsignals, said subset only including control signals determined asreliably received; and transmit in the uplink direction, using the atleast two uplink transmit antennas, while applying the derived one ormore parameters to control the uplink transmit diversity operation. 2.The user equipment of claim 1, wherein the control signals includes oneor more of the following types of control signals: transmit powercontrol commands sent to the user equipment for controlling uplinktransmit power of the user equipment; Hybrid Automatic Repeat Request(HARQ) Acknowledgement/Negative Acknowledgement (ACK/NACK) signals sentto the user equipment for controlling HARQ retransmissions by the userequipment; and control signals for explicit control of the userequipment transmit diversity parameters.
 3. The user equipment of claim1, wherein controller circuitry for estimating a received signal qualityis arranged to estimate the received signal quality based on one or moreout of: Signal to interference ratio (SIR); Signal to interference plusnoise ratio (SINR); bit error rate; block error rate; SDU error rate;Frame error rate; symbol error rate.
 4. The user equipment of claim 1,wherein the one or more parameters comprise one or more of thefollowing: relative phase; relative power; relative frequency; timing;relative amplitude; absolute power; of signals transmitted on transmitdiversity branches.
 5. The user equipment of claim 4, wherein the uplinktransmit diversity operation is transmit diversity beamforming and theone or more parameters comprise relative phase and amplitude of signalstransmitted on transmit diversity branches.
 6. The user equipment ofclaim 4, wherein the uplink transmit diversity operation is switchedantenna uplink transmit diversity and the one or more parameterscomprise amplitude or power of signals transmitted on transmit diversitybranches.
 7. The user equipment of claim 1, wherein the user equipmentis arranged to perform uplink transmissions using less than all transmitantennas in said set of at least two uplink transmit antennas if nocontrol signal in said set of control signals is determined as reliablyreceived.
 8. The user equipment of claim 1, wherein the user equipmentis arranged to switch to a single transmit antenna transmission mode inwhich uplink transmissions are performed using a single antenna in saidset of at least two uplink transmit antennas if no control signal insaid set of control signals is determined as reliably received.
 9. Theuser equipment of claim 1, wherein the user equipment is arranged toperform uplink transmissions using less than all transmit antennas insaid set of at least two uplink antennas if the set of control signalsconsists of a number of N control signals out of which less than anumber of M control signals are determined as reliably received, M<N.10. The user equipment of claim 1, wherein the user equipment isarranged to switch to a single transmit antenna transmission mode inwhich uplink transmissions are performed using a single antenna in saidset of at least two uplink transmit antennas if the set of controlsignals consists of a number of N control signals out of which less thana number of M control signals are determined as reliably received, M<N.11. The user equipment of claim 1, wherein the user equipmentconnectable with multiple radio links and where the user equipment isadapted to receive downlink signals or commands from more than one radiolink, the user equipment comprising: controller circuitry arranged touse only downlink commands whose quality is above threshold from eachradio link for deriving or adjusting one or more uplink transmitdiversity parameters.
 12. The user equipment of claim 1, wherein theuser equipment is configured to explicitly notify the network about thestatus of inappropriate setting of parameters for the uplink transmitdiversity due to unreliable received commands.